You might be referring to the unreal editor limitation on how big a float can be entered into UI fields. The max value you can type into an unreal editor UI float field is 9.223372036854776x10^18 (9223372036854775808.0), half of 18,446,744,073,709,551,615.
But unreal blueprints can still hold float values up to the float limit (3.4028235x10^38).